The present invention relates to a curable composition for paint which has excellent acid resistance, water resistance, scratch resistance and weathering resistance. More particularly the invention relates to a curable composition suitable for use as a paint for top coatings of automobiles, industrial machines, steel furniture, outer walls of buildings, household electric appliances, plastics, and the like.
Up to the present time, resins for paint used for coating automobiles, outer walls of buildings, industrial machines, steel furniture, plastics and the like, have been mainly melamine resins such as alkyd melamine and acrylic melamine.
However, these melamine resins have problems in that noxious formalin is generated during the curing process and that the cured films are damaged by acid rain as a result of their poor acid resistance. In particular, the problems of acid rain resulting from air pollution have become serious recently, with the occurrence of etching, blushing and stains being observed.
In order to solve these kinds of problems, the present inventors had investigated the use of a vinyl copolymer having a silicon atom to which a hydrolyzable group is bonded (hereinafter also referred to as "hydrolyzable silyl group"), or a blend system thereof with an acrylic polyol, for paint. As a result, it was found that this kind of copolymer or blend system can solve the above-mentioned problems and can give a curable composition for paint which has excellent acid resistance and weathering resistance, as described in prior patent applications (see, for example, Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 132977/1988 and the like).
The above-mentioned vinyl copolymer having a hydrolyzable silyl group, and the blend system thereof with an acrylic polyol, have superior acid resistance and weathering resistance compared to melamine resins such as acrylic melamine and alkyd melamine. This is because their silyl group and alcoholic hydroxyl group form a stable siloxane bond or siloxy bond to cure the copolymer or blend system.
The hydrolyzable silyl group shown by the formula (I): ##STR1## (wherein R.sup.1 is an alkyl group having from 1 to 10 carbon atoms; R.sup.2 is a hydrogen atom or a monovalent hydrocarbon group selected from an alkyl group, an aryl group and an aralkyl group each having 1 to 10 carbon atoms; and a is 0, 1 or 2) is incorporated into the main chain of the copolymer through a Si--C bond. As a result, copolymers having this kind of hydrolyzable silyl group have excellent acid resistance and weathering resistance.
In comparison to the system which does not have an acrylic polyol, the blend system of a vinyl copolymer having a hydrolyzable silyl group with an acrylic polyol has both superior heat-curability and appearance. In addition, there is the further advantage that the amount of the compound having a hydrolyzable silyl group, which is a relatively expensive raw material, can be reduced, leading to the production of a curable composition at low cost. However, the blend system has a disadvantage in that it has a somewhat inferior water resistance and acid resistance. Furthermore, in the blend system, improvement in the compatibility of the vinyl copolymer having a hydrolyzable silyl group with the acrylic polyol is necessary, so that the molecular weight of each resin, the composition of the copolymerizable compounds and the blending ratio are restricted.
The present invention has been realized in consideration of these kinds of problems inherent in the prior art, and accordingly, an object of this invention is to provide a curable composition for paint having excellent heat-curability, appearance, acid resistance and water resistance, and which is produced at low cost.